Fix AVX-512 round function

[AngryLoki]

Description

This PR fixes vfloat16 round function. Intrinsic _mm512_roundscale_ps was
used incorrectly, and caused failure on Zen4 CPU.

/var/tmp/portage/media-libs/openimageio-2.5.5.0-r1/work/OpenImageIO-2.5.5.0/src/libutil/simd_test.cpp:1579:
FAILED: round(F) == mkvec<VEC>(std::round(F[0]), std::round(F[1]), std::round(F[2]), std::round(F[3]))
	values were '-1.5 0 1.5 4 -1.5 0 1.5 4 -1.5 0 1.5 4 -1.5 0 1.5 4' and '-2 0 2 4 -2 0 2 4 -2 0 2 4 -2 0 2 4'

In old code _mm512_roundscale_ps (a, (1<<4) | 3) meant the following:

[0001] - Number of fixed points to preserve
[0] - Use MSCSR exception mask
[0] - Select mode from imm
[11] - Truncate mode

Effectively enabling rounding to nearest 0.5, not to integer.

References:

• https://www.felixcloutier.com/x86/vrndscalepd#fig-5-29
• https://stackoverflow.com/questions/50854991/instrinsic-mm512-round-ps-is-missing-for-avx512

Tests

• This fixes test_simd

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[linux-foundation-easycla]

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• ✅ login: AngryLoki (e114063)

[lgritz]

This looks right to me, thanks for the fix!